Chex Quest 2

In the sequel to Chex Quest, you again play the square cereal man. This time, your Flemoids have taken over your cities, and it's up to you to stop them. Using your arsenal of weapons from your first adventures, you travel to through the city all the way to the sewers, where you must face the head Flemoids themselves.

This game was downloadable from the Chex Quest website. It could be classified as an add-on or mod, since it uses the same executable files to run the original game, but it technically isn't since it's a whole game.